Radio frequency coupler and attenuator



June 9, 1964 w. s. GEISLER, JR 3,135,964

RADIO FREQUENCY COUPLER AND ATTENUATOR original Filed May 12, 1954 2 sheets-sheet 1 /Nl/E/v TOR W/LSON 5. GE/SLER,

1 l/ a a MV,

v A Q A 7' TORNE Y Original Filed May 12, 1954 June 9, 1964 w. s. GEISLER, JR 3,136,964

RADIO FREQUENCY COUPLER AND ATTENUATOR 2 Sheets-Sheet 2 ATTORNEY United States Patent AO 3,136,964 i RADIO FREQUENCY COUPLER AND ATTENUATOR Wilson S. Geisler, Jr., Atherton, Calif., assignor, by mesne assignments, to High Voltage Engineering Corporation,

Burlington, Mass., a corporation of Massachusetts Original application May 12,1954, Ser. No. 429,265, now Patent No. 2,939,045, dated May 31, 1960. Divided and this application Oct. 30, 1959, Ser. No. 849,835

Claims. (Cl. S33-10) This invention relates to electron discharge devices and more particularly, to traveling wave tubes operable at microwave frequencies as broad band amplifiers.

Since traveling Wave tubes are of comparatively recent vintage, little Work has been done except on theoretical and experimental investigation. As a consequence, the traveling Vwave tubes themselves, the arrangement for coupling the microwave frequency energy into and out of the tubes, and the arrangement for focusing the electron beam, have been somewhat inefficient and in most cases, excessively cumbersome. f

Accordingly, it is an object of the present invention to provide a traveling Wave tube arranged in combination with radio frequency couplers and focusing means as a compact and eicient unit or package.

A feature of the invention involves the provision of a traveling wave tube unit having integrated means for establishing the required external electrical connections with utmost facility.

Another feature involves the provision of an improved arrangement for physically mounting a traveling wave tube or similar electron discharge device.

A further feature is the provision of a novel and particularly effective arrangement for coupling radio frequency energy intoand out of a traveling wave tube.

A further feature involves the provision of an effective yet simple attenuator for utilization'with such a tube.

Additionally, a feature of the invention is directed to the actual structural features of the tube which facilitate both the assembly and operation.

Other features of the invention, as well as the advantages obtained therefrom will become readily apparent from the following description taken in conjunction with the accompanying drawings wherein:

FIGURE l is a side View in elevation of a traveling wave tube unit or package embodying the present invention, with a portion of the ,outer housing broken away to illustrate interior details of the construction;

l FIGURE 2 is a longitudinal sectional View of the traveling wave tube before its incorporation in the package;

FIGURE 3 is a transverse section of the traveling Wave tube unit taken along line 33 of FIGURE l and illustrating the manner in which the coupling and focusing rneans are joined physically to the tube;

VFIGURE 4 is a fragmentary section taken along line V4--f4 of FIGURE 3; and p i FIGURE 5 is a longitudinal section of a central portion of the unit showing details of an attenuator structure.

As most clearly illustrated in FIGURE 2, the travel- 'ingwave tube 'constructed in accordance with the present invention includes an elongated glass body lll of small tubular cross section throughout most of its length but'enlarged at one end as indicated at 11 to enable mounting of an electron gun 12. Within the glass tube a slow-wave structure in the form of a metal helix 13, is disposed to provide the desired interaction between lthe electrons which emerge as a beam from the electron guni12 and the radio frequency Wave applied to the khelix 13 in a manner to be described hereinafter.

structurally, the helix 13 is provided at one end with a small hook 14 and at the other is spotwelded to a metal ICC base 15 which preferably takes the form of a short cylindrical stub 16 substantially equal in diameter to the helix and having an annular ring 17 brazed to its exterior. The described tubular glass body 10 is formed upon an accurately machined mandrel so that its inner diameter closely corresponds to the outer diameter of the helix 13 and thus enables slidable insertion, yet subsequent supportof the latter. The hooked end 14 of the helix 13 is inserted into the body 10 at its enlarged end 11 and is pushed therethrough until the annular ring portion 17 of the described helix base 15 cornes into engagement with the shoulderformed at the enlarged end 11 of the glass body. The hooked end 14 of the helix is then pulled and brought into hooked engagement over the other end ofthe glass body 10 and is `subsequently engaged by a small metal tubulation 1S sealed to the end of the body. Such engagement maintains the helix 13 permanently n its slightly stretched position and establishes electrical connection between the helix and the tubulation. `The geometry of the helix 13 is such that when secured in the manner described, the phase velocity of the applied radio frequency wave along the axis corresponds very closely to the velocity of the electrons injected from the electron gun 12.

The electron gun 12 is generally of conventional construction including a cathode 20 supported by means of an annular ceramic spacer 21 on the interior of a cupshaped body 22. The side Wall of the gun body 22 is formed of a pair of copper cylinders 23, 23 having lateral flanges brazed together and to the end of one cylinder 23 a first anode 24 in the form of a copper cup is brazed so that a central opening therein lies` adjacent and axially aligned with the cathode 20. The electron gun 12 is mounted so that the cathode 20 and rst anode 24 arey aligned with the axis of the helix 13, such mounting being accomplished by means of a copper cylinder 25 secured at one end to the exterior of the gun body 22, as by brazing, and being supported at its other end in vacuum-tight relation on the end of the enlarged portion 11 of the glass body 10. The base of the cupshaped gun body 22 is formed by a ceramic disc 26 which is sealed'to the end of the cylinder 23 of the gun body 22 and supports in vacuum-sealing relation wires 27, 28, 29 which extend therethrough to enable application of voltage to the cathode 20, to a filament 30 for heating the cathode, and additionally, to a conventional getter 31, which can be'red subsequent to evacuation of the traveling wave tube to purge all remaining impurities.

Evacuation of the tube is accomplished, in accordance with the present invention, through the previously described tubulation 18 at its distal end. After the tube has been pumped through this tubulation, it is carefully pinched off, as shown in the drawing, to subsequently serve as a collector for the spent electrons which have traversed the full length of the tube.`

Brazed to the exterior of the gun body 22 is an annular disc 32 which controls the longitudinal disposition of the traveling Wave tube Within a cylindrical housing 33 that is adapted to receive the tube.` In accordance with the present invention, this housing 33 not only serves as a protective cover for the traveling Wave tube but also mounts within its interior the means for focusing the electron beam within the traveling wave tube and the means for coupling radio frequency energy into and out of the tube. The arrangement is such that the tube, when inserted, is held in proper spaced relation to the focusing means.

Asbest shown in FIGURE .1, this housing 33 constitutes an elongated metal tube as of steel, such metal tube being provided with a number of openings enabling radio frequency input and output connections to be made therethrough and additionally, enabling the insertion of cap screws 34. At each of three points longitudinally of the tubular housing 33, three of the cap screws 34 are inserted in the openings which are preferably disposed at circumferential intervals of approximately 120 degrees. Each screw 34 is adapted to enter a registering threaded bore extending radially through the dividing portions 3S on a partitioned brass spool 36 upon which are wrapped wires to form coils 37 that constitute the aforementioned focusing means for the traveling wave tube. This spool 36, after slidable insertion into the tubular housing 33 so that the threaded bores thereof register with the described openings in the housing, is then clamped securely Within the housing by the cap screws 34.

As best shown in FIGURES 3 and 4, the cap screws 34 project only a short distance into the threaded bores of the brass spool, and set screws 38 projecting inwardly from the bores are adapted to engage and support through such engagement, small tubular nonmagnetic metal housings 39 for axially spaced input and output couplers and a generally similar housing 39 for an attenuator disposed intermediately thereof. These set screws 38 are held in position by lock screws 40 disposed atY an intermediate position in the threaded bores in the spool 36.

Within each coupler housing 39, a tubular electrostatic shield-41 is supported and itself encompasses a coupling helix 42 of a diameter such that a traveling wave`tube can be inserted thereinto. The pitch of the coupling helix 42 is arranged so that the phase velocity of radio frequency energy applied thereto is substantially equivalent to the phase velocity of the helix 13 within the inserted traveling wave tube. The electrostatic shield 41 is maintained in spaced relation to the coupling helix 42 by tubular insulation 43 such as Teflon, (Tefion is the trade name of the du Pont Chemical Co. for tetrafluoroethylene) so as to constitute, in effect, a section of coaxial transmission line having an impedance determined by the length of the section. The lengthfis chosen, in accordance with the present invention, to produce an impedance value equivalent to that of the feed line for the input or output couplers. This feed line, as shown in FIGURE 3, constitutes a coaxial cable 44, the inner conductor 45 of which is soldered or otherwise suitably connected to one end of the coupling helix 42 and passes outwardly through a radial opening in the coupler housing 39 and the outer conductor 46 of which is soldered or otherwise suitably connected to the coupler housing 39. The tubular shield 41, the tubular insulation 43, and the coupling helix 42, which may be considered as forming an assembly, are maintained within the coupler housing 39 by means of packing glands at the ends thereof. Each gland constitutes a rubber Washer 47 which is pressed against the end of the coupling helix 42 and the surrounding insulation 43 and shield 41 by an annular nut 48, which may be screwed into the threaded interior ends of the coupler housing 39.

` The two couplers and the attenuator are adapted to closely encompass an inserted traveling wave tube and upon tightening of the packing nuts 48, the washers 47 are squeezed so that they resiliently engage the glass body 10. Since the couplers and the attenuator are supported by the described set screws'38 held by the lock screws 40, the disposition of the traveling wave tube Within the outer cylindrical housing 33 and more particularly, within the focusing coils 37 may be securely maintained. To adjust the axial disposition of the traveling wave tube, one or more of the cap screws 34 and the lock screws 40v are removed and the respective set screws 38 are turned to effect displacement of all or merely one portion of the traveling wave tube, as desired, to place the tube in properly spaced relation to the focusing coils 37. Thereafter, the lock screws 40 and the cap screws 34 are reinserted to hold the unit securely in its adjusted position.

The intermediately mounted attenuator is similar in construction to the input and output couplers, like parts being'indicated by like numerals with an added prime notation. They differ only in the facts that a layer 49 of aquadag or other lossy material is applied to the exterior surface of the insulation 43 and of course, that any ex terior radio frequency connection is omitted. This type of attenuator is to be preferred over the normal or conventional type, which merelyv constitutes a coating of aquadag applied to the exterior of the glass tube, in that it provides effectively a more concentrated attenuation and eliminates the so-called tube effect which deleteriously affects the phase velocity of the radio frequency wave within the traveling wave tube.

The coaxial cables 44 connected to the input and output couplers pass radially outward from the coupler housings 39 through radial bores in the brass spool 36 and connection between the outer conductor of lthe cable 44 and the housing 33.

After the traveling wave tube has been inserted into the housing 33 so as to be encompassed by each of the input and output couplers, as Well as by the intermediate attenuator, and the packing nuts 4Srare tightened to provide the described resilient support, the previously described annular disc 32 on the gun body 2.2 is then secured by screws 52 tothe ends 'of the spool 36, supporting the focusing coils 37, thus to maintain as previously mentioned, the longitudinal disposition of the traveling Wave tube within the housing 33. Thereafter, one end of the outer housing 33 is closed by a metal cap 53 and the other end is provided with an insulated socket 54 to which leads from the various electrical elements of the traveling wave tube may be connected to thereby facilitate connection of exterior electrical power to energize the tube.

The described first anode 24 is tied through the copper side wall of the electron gun body 22 and the mounting disc 32 to the outer housing 33 which is, as previously mentioned, maintained at ground potential. The cathode and heater wires 28, 29 which emerge from the ceramic base 26 of the 'electron gun 12 are secured to suitable leads which pass between the focusing coils 37 and the outer housing 33 for connection individually to prongs (not shown) in the insulated socket 54 at the other end of the housing, the cathode Wire 27 being adapted for connection to a D.C. supply which'maintains it at 300 volts negative relative to the described first anode 24. Another wire 55 connected between the socket 54 and the collector 18 at the end of the traveling Wave tube is adapted to supply a positive potential of approximately 2500 volts to the collector 18 and to the helix 13 and its base 15 which, in effect, provides the second anode for the traveling wave tube. When the power is supplied to these connections by a suitable plug (not shown) and the radio frequency energy is supplied to the input coaxial connector Si?, the beam in its traverse of the traveling wave tube interacts with the radio frequency wave so applied to amplify the same in the well-known manner and the amplified output is extracted through the output coaxial cable 44 and its connector 50. The intermediately positioned attenuator acts effectively to preclude oscillation within the tube in a Well-known manner, the incorporated shield 41 serving to enhance the effect of the aquadag layer 49.

It is apparent that the traveling wave tube unit, as described requires only simple radio frequency input and output connections to the coaxial connectors S0 and direct current voltage connections to the insulated socket 54 on the housing 33 to enable operation. Consequently, the use of this traveling wave device is facilitated and because of the protected resilient mounting of the fragile glass body 10 of the tube, installation is enabled at points where vibration or other factors would normally prohibit such use. Additionally, because of the steel construction of the housing 33, stray magnetic fields cannot enter to defocus the beam in the traveling wave tube.

5 v Y Various modications and alterations may obviously be made in either the construction of the unit or the traveling Wave tube itself Without departing'from the spirit of the present invention. For example, the mounting arrangement embodied in the described unit might Well be utilized for supporting a backward Wave oscillator of generally similar construction. Consequently, the foregoing descriptionand the accompanying drawings are `to be considered as purely exemplary and not in a limiting sense, the scope of the invention being indicated by the appended claims. v

This application is a divisionV of my earlier iled co- Vpending application, Serial Number 429,265, filed May 12, 1954, now Patent Number 2,939,045, issuedflvlay 3l, 1960.

VI claim: l

1. A radio `frequency coupler for a traveling Wave tube having an elongated tubular glass body, said radiofrequency coupler comprising a tubularelectrostatic shield, a conductive helix adapted to encompass and engage the elongatedf tubular 'body of the traveling Wave tube, tubular insulation surrounding said helix and spacing said helix from said shield to effect an impedance, said shield, helix and insulation being all of the same length and overlapping `one another, a tubular outer housing for said helix, insulator and shield forming an assembly and overhanging each end, and resilient annular means` cooperating with the ends of said housingto resiliently squeeze the assembly Within said housing for resiliently engaging f the tubular glass body of said traveling Wave tube, and a coaxial cable the inner conductor of which is connected to one end of said helix and the outer conductor is connected to said housing.

2. A radio frequency coupler for a traveling wave tube having an elongatedv tubular glass body containing a metallic helix, said radio frequency coupler comprising y y a tubular electrostatic shield, a conductive helix adapted to encompass and engage the elongated tubular body of the traveling wave tube, said helix having a pitch arranged so that the phase velocity of the radio frequency energy applied is substantially equivalent to the phase velocity of the metallic helix Within the tube, tubular insulation surrounding said helix and spacing said helix from said shield to effect an impedance, said shield, helix and insulation being all of the same length and coaxially formingan assembly, a tubular outer housingfor said asseml bly and overhanging each end, said ends being internally f threaded, resilient annular washers Within said housing at each end, annular means cooperating with the threaded ends of saidhousing to adjustably and resiliently squeeze said assembly Within said housing `for resiliently engaging the elongated glass body of said traveling wave tube, and aY coaxial cabledisposed at right angles to the ,axis of said .v housing with the inner` conductor connected to one end v6 l of the conductive helix and the outer conductor connected to the said housing.

3. A radio frequency coupler according to claim 1 wherein the insulator is tetrauoroethylene.

, 4. An attenuator for mounting on a traveling Wave tube having an elongated tubular body containing a metallic helix,` said attenuator comprising in combination a conductive helix adapted to closely encompass the tube, tubular insulation around said helix, a layer of lossy material supported on said tubular insulation and a tubular electrostatic shield surrounding said layer of lossy material, said helix, insulator,'lossy material .and shield all being an assembly of substantially equal length, a tubular housing for said assembly overhanging it at both ends, annular resilient washers'within the housing at each end, and annular means engaging within and cooperating with the overhanging ends of said housing to resiliently squeeze said annular washers to resiliently engage and hold the tube.

5. A Vradio frequency coupler for a traveling wave tube having an elongated glass tubularkbody with a metal helix therein, adapted for coaxial mounting thereon, comprising la Vcoupling helix having af diameter such that the traveling Wave tube mayy be inserted therethrough and having a pitch arranged so that the phase velocity of radio frequency energy applied is substantially equivalent to the phase velocity of the helix Within the elongated tube, tubular insulation surrounding said helix and of substantially equal length therewith, a tubular electrostatic shield surrounding said insulation and of substantially equal length therewith, a non-magnetic tubular coupler housing for the coaxial assembly of said helix, insulator and shield, overhanging the same at each end, annular packing glands within said housing at each end, annular closing nuts at each end of said housing for exerting pressure against said resilient glands, and a coaxial cable feed line, one conductor of which is attached to the conductive helix and the other conductorof Which is attached to the coupler housing, the length of the tubular insulation being selected to produce an impedance value equivalent to Vthat of the coaxial feed line. Y

References Cited in the le of this patent UNITED STATES PATENTS vWolkstein et al. Feb. 6, 

1. A RADIO FREQUENCY COUPLER FOR A TRAVELING WAVE TUBE HAVING AN ELONGATED TUBULAR GLASS BODY, SAID RADIO FREQUENCY COUPLER COMPRISING A TUBULAR ELECTROSTATIC SHIELD, A CONDUCTIVE HELIX ADAPTED TO ENCOMPASS AND ENGAGE THE ELONGATED TUBULAR BODY OF THE TRAVELING WAVE TUBE, TUBULAR INSULATION SURROUNDING SAID HELIX AND SPACING SAID HELIX FROM SAID SHIELD TO EFFECT AN IMPEDANCE, SAID SHIELD, HELIX AND INSULATION BEING ALL OF THE SAME LENGTH AND OVERLAPPING ONE ANOTHER, A TUBULAR OUTER HOUSING FOR SAID HELIX, INSULATOR AND SHIELD FORMING AN ASSEMBLY AND OVERHANGING EACH END, AND RESILIENT ANNULAR MEANS COOPERATING WITH THE ENDS OF SAID HOUSING TO RESILIENTLY SQUEEZE THE ASSEMBLY WITHIN SAID HOUSING FOR RESILIENTLY ENGAGING THE TUBULAR GLASS BODY OF SAID TRAVELING WAVE TUBE, AND A COAXIAL CABLE THE INNER CONDUCTOR OF WHICH IS CONNECTED TO ONE END OF SAID HELIX AND THE OUTER CONDUCTOR IS CONNECTED TO SAID HOUSING. 